1. Field of Invention
The present invention is directed to a semiconductor device having a memory array comprising a word line with reduced resistance and a method of manufacturing the same.
2. Description of the Related Art
Nowadays, semiconductor devices are subject to increasing demands to shrink the design rules. To moot such demands, semiconductor devices, typically a flash memory employing a NOR memory cell region requires narrower spaces between the word lines of each memory cell. This leads to increase in the aspect ratio, indicative of the relation of word line height relative to word line gap, thus increasing the difficulty of filling the spaces between the word lines with insulating films.
The following problems are observed in NOR flash memories, particularly in NOR flash memories of a substrate erase type that form multiple silicon substrate contacts between the word lines. In the event a void occurs in the material filling the spaces between the word lines, a leak path is generated between the neighboring contacts via the void, thereby shorting the neighboring contacts and ultimately resulting in defective cells. To reduce the aspect ratio upon filling the word line spaces, a method may be employed in which the word line height is lowered by reducing the thickness of the word line. However, thinning the word line will result in the reduction of cross-sectional area taken in the direction of the conductive path of the word line, which in turn increases the resistance of the word line and possibly reduce the read speed of the memory cell. Such aspect is a negative factor in employing this method.
JP-A-H05-82467 discloses a conventional method to overcome the difficulty of forming contact holes imposed by design rule shrink. The disclosure indicates a method of reliably defining the contact holes in different vertical positions by arranging either of the contact holes in a rectangular profile.
The disclosed construction may prevent the increase of resistance of the contact holes but is yet unsuccessful in reducing the electrical conductivity of the word line itself.